1. Field of the Invention
This invention pertains generally to an apparatus and method for generating a desired shape for baseband wireless communication. More particularly, the invention relates to an apparatus and method for generating a baseband signal from an input pulse.
2. The Prior Art
The prior art teaches pulse shaping techniques for baseband systems. One of the prior art pulse generators is disclosed in U.S. Pat. No. 5,146,616 issued to Tang et. al. and describes an ultra wideband radar transmitter employing synthesized short pulses. The patent provides an impulse radar apparatus employing multiple transmitters and receivers. Each transmitter operates in a continuous wave mode in which all the spectral components are transmitted together. To achieve phase coherency in all transmitters, a master oscillator is coupled to a harmonic generator which provides all the required spectral components to drive a plurality of final amplifiers. The signals from the final amplifiers are coupled by a plurality of duplexers to a broadband multiplexing antenna which forms a high gain beam. The patent describes generating and radiating a pulse train that is representative of a predetermined desired radar signal to be synthesized in space. Rather than generating a signal in real time by switching a radio frequency transmit signal on and off, in the patent the individual spectral components are generated and transmitted instead. The spectral components have uniform amplitudes and are evenly spaced apart.
Another prior art pulse generator is disclosed in U.S. Pat. No. 5,274,271 issued to McEwan which describes an ultra-short pulse generator. The ultra-short pulse generator is an inexpensive circuit that generates ultra-short, 200 picosecond, and high power 100 kW, pulses suitable for wideband radar and other wideband applications. The patent discloses a driver circuit in which the driver circuit furnishes a high-voltage, step-like input pulse that increases in voltage, and decreases in duration as it passes through the transmission line. The transmission line comprises a series of stages with each stage having an inductor and one or more diode-capacitors. Each stage sharpens and raises the voltage peak of the step-like input from the driver circuit. The length of the transmission line is chosen so that the voltage step at the final stage induces transit time avalanche breakdown in a diode that is placed in a suitable breakdown circuit at the output of the nonlinear transmission line. This advantageously sharpens the output pulse of the circuit even more, making it short enough for use in many broadband applications.
Another prior art pulse generator is disclosed in U.S. Pat. No. 5,307,079 issued to Ross which describes a short pulse microwave source with a high pulse repetition frequency and low power drain. The patent describes a high voltage, very short pulse, microwave radiating source using low-cost components, and capable of operating at high pulse repetition frequencies. The source is activated by an ordinary video trigger which is capable of driving TTL logic. A trigger will cause a chain of N (where N may be 12 or greater) avalanche transistors connected in a Marx generator configuration to threshold. Thus, resulting in a 1,200 volt or greater baseband pulse having a rise time of less than 100 ps and a duration of about 3 ns driving the input port of a dipole antenna. The Marx generator configuration permits one to charge a bank of capacitors in parallel from a low battery voltage and then discharge them in series creating a high voltage pulse. The circuit assures that the capacitors are charged during a short interval before application of the main avalanche trigger, and the power supply is disconnected just prior to triggering the modified Marx generator.
Another prior art pulse shaping-generator is disclosed in U.S. Pat. No. 5,319,218 issued to Kim et. al. which describes pulse sharpening using an optical pulse. The patent describes a monolithic, photoconductive impulse generating device having metalized electrodes on two opposing surfaces. A laser light is optically connected to each surface through a fiber optic cable where the cable leading to one surface differs in length from the cable leading to the other surface by some predetermined amount. As such light energy passing through the longer cable will arrive at its respective surface at some predetermined time after the light passing through the shorter cable. Consequently, the energy discharge that is triggered by the light passing through the shorter cable will be abruptly terminated by the light passing through the longer cable.
Another prior art pulse shaping generator is disclosed in U.S. Pat. No. 5,815,537 issued to Janssen which describes a wireless digital communication device, and a pulse shaping network. The pulse shaping network comprises a shift register to which data to be modulated are fed. Outputs of the shift register are connected to control inputs of switched weighted current sources which are summed. The amplitude of the shaped pulse can be accurately determined by adjusting the input voltage of the pulse shaping network.
These aforementioned approaches and examples apply pulse shaping techniques to baseband systems. The short pulses are generated by having multiple transmitters operating at the same time (as described in Tang et. al.), by sharpening pulses with a nonlinear transmission line (as described in McEwan), with avalanche transistors (as described in Ross), with optical pulses (as described in Kim), or with a weighted current network (as described in Janssen). However, none of these approaches or examples teaches an apparatus or method which is presented with a data pulse and generates a series of delayed short pulses that are combined to generate a baseband signal which closely approximates a filter""s transfer function.
Accordingly, it is an object of the invention is to provide an apparatus to be used for wireless communication that generates a series of delayed short pulses which generate a baseband signal which closely approximates a filter""s shape.
Another object of the invention is to provide a baseband signal generator apparatus and method for shaping a pulse for transmission.
Another object of the invention is to provide a baseband signal generator capable of operating at frequencies in the gigahertz range.
Another object of the invention is to provide a baseband signal generator that uses pull-up circuits and pull-down circuits to generate output signals in the gigahertz range.
Other objects, together with the foregoing are attained in the exercise of the invention in the following description and resulting in the embodiments described with respect to the accompanying drawings
The present invention is an apparatus and method which maximizes the energy transfer through a filter by matching the shape of the signal to the filter""s transfer function. The filters transfer function is approximated using inexpensive digital circuits which generate a series of delayed short pulses that are combined to generate an output baseband signal that approximates the shape of the filters transfer function.
In its preferred embodiment, the present invention is a baseband signal generator that generates an output signal from data pulses. The baseband signal generator of the present invention comprises a pull-up circuit and a pull-down circuit which generate the baseband output signal that approximates the shape of the filter transfer function. The pull-up circuit includes a transistor drive system having a pnp bipolar transistor which produces positive going signal excursions. The pull-down circuit also includes the transistor drive system having a npn bipolar transistor which produces negative going signal excursions. The composite of the positive going signal excursions and the negative going signal excursions generate the output baseband signal that approximates the shape associated with the filter transfer function.
By way of example and not of limitation, the output baseband signal may have a spectral content bandwidth which matches the filter bandwidth between 2.5 GHz to 5.0 GHz. Note that in the preferred embodiment, the filter is an antenna transmitting signals between 2.5 GHz and 5.0 GHz. At these operating frequencies, the capacitance at the base-emitter junction of the bipolar transistors prevents the bipolar transistors from rapidly turning off. To ensure rapid turnoff of the transistors, the present invention generates xe2x80x9cturn offxe2x80x9d signals to discharge the capacitance at the base-emitter junction and turns off the transistor.
Therefore, the pnp pull-up transistor is operated using a pair of signals including a pull-up turn-on signal and a pull-up turn-off signal. The pull-up turn-on signal activates the pnp transistor and generates an output pulse. The pull-up turn-off signal is presented to the pnp transistor which discharges the base emitter capacitive charge to turn off the pnp transistor. The combination of the pull-up turn-on signal and the pull-up turn-off signal produces a positive going signal excursion for the output baseband signal.
The pull-down circuit operates in a similar manner as the pull-up circuit. The npn pull-down transistor is operated using a pair of signals including a pull-down turn-on signal and a pull-down turn-off signal. The pull-down turn-on signal activates the npn transistor and generates an output pulse. The pull-down turn-off signals discharge the base emitter capacitance of the npn transistor to turn it off. In combination the pull-down turn-on signal and pull-down turn-off signal generates negative going signal excursions for the output baseband signal. The composite of the positive going signal excursions and negative going signal excursions generate a baseband output representing a binary xe2x80x9c1xe2x80x9d.
The pull-up circuits and pull-down circuits also include a pulse generator system. The pulse generator system is comprised of one or more pulse generators. In operation, the pulse generating system presents the rising edge of an input transmit pulse to the one or more pulse generators. The pulse generator system generates output signals that are presented to the transistor drive system as pull-up turn-on signals, pull-up turn-off signals, pull-down turn-on signals, and pull-down turn-off signals.
Each pulse generator includes one or more pairs of coupled edge delay circuits. The edge delay circuits function in pairs where the first edge delay circuit generates the leading edge for the delayed pulse signal and the second edge delay circuit generates the trailing edge for the delayed pulse signal. Each edge delay circuit may comprise a switched bank of capacitors that provide a programmable edge delay. The outputs from each pair of edge delay circuits are combined to produce a composite series of delayed output pulses. The delayed output pulses from the one or more pulse generators are presented as pull-up turn-on signals, pull-up turn-off signals, pull-down turn-on signals, and pull-down turn-off signals to the transistor drive system having the pnp and npn bipolar transistors described above.
It shall be appreciated by those with ordinary skill in the art that the pnp transistors and npn transistors may be external or internal to the digital circuit. Additionally, an alternative embodiment exists where pnp transistor of the pull-up circuit and the npn transistor of the pull-down circuit are not used and the pull-up signals and pull-down signals are coupled to a load. More particularly, the current source line and sink line are tied together to a node which drives a load, such as an antenna, in a current drive configuration, using a current to produce an electric field on the antenna.